In Qiskit#8201 I added a release note as part of the PR which documented the
change in behavior. However, I accidentally committed this file in the
wrong location (by running reno new outside of the repo root). This
meant the file was never actually included in the release notes for the
0.21.0 release. This commit corrects this oversight and moves it back to
the proper location.

However, since this isn't my first time making this mistake and I can
expect that others will make it too in the future. This commit also adds
a new script to detect this and raise an error when release notes are
present outside of the proper location. By running this as part of lint
jobs we'll block this mistake from happening again.